The 2018 Mw 6.8 Zakynthos, Greece, earthquake – strike-slip and thrust faulting in shallow subduction

2020 ◽  
Author(s):  
Efthimios Sokos ◽  
František Gallovič ◽  
Christos P. Evangelidis ◽  
Anna Serpetsidaki ◽  
Vladimír Plicka ◽  
...  
Keyword(s):  
Numerical Models ◽  
Waveform Inversion ◽  
Broad Band ◽  
Strong Motion ◽  
Aftershock Sequence ◽  
Strike Slip ◽  
Plate Motion ◽  
Ocean Bottom ◽  
Ocean Bottom Seismometer ◽  
Fault Interaction

<p>On October 25, 2018, at 22:54 UTC, an Mw 6.8 earthquake occurred southwest of Zakynthos island in the Ionian Sea. This is an area with different styles of faulting and the locus of strong events thus ideal for fault interaction studies. The 2018 Zakynthos earthquake was recorded by broad-band and strong-motion networks and provides an opportunity to resolve such faulting complexity. We used waveform inversion and backprojection of strong motion data, partly verified by co-seismic GNSS data, too. The aftershock sequence was relocated, and the moment tensors of the strongest events were evaluated. Stress inversion shows that the region is under sub-horizontal southwest-northeast compression, enabling mixed thrust- and strike-slip faulting. Based on detailed waveform inversion studies, we conclude that the 2018 mainshock consisted of two fault segments: a low-dip thrust, and a dominant, moderate-dip, right-lateral strike slip, both in the crust. This model explains the observed large negative CLVD component of the mainshock. Slip vectors of both ruptured segments, oriented to SW, are consistent with plate motion in the area. The sequence can be explained in terms of trench-orthogonal fractures in the subducting plate and reactivated faults in the upper plate. The 2018 event, and an Mw 6.6 event of 1997, occurred near three localized swarms of 2016 and 2017. Future numerical models of the slab deformation and ocean-bottom seismometer observations may illuminate possible relations between earthquakes, swarms and fluid paths in the region.</p>

The 2018 Mw 6.8 Zakynthos, Greece, Earthquake: Dominant Strike-Slip Faulting near Subducting Slab

2020 ◽  
Vol 91 (2A) ◽  
pp. 721-732 ◽  
Author(s):  
Efthimios Sokos ◽  
František Gallovič ◽  
Christos P. Evangelidis ◽  
Anna Serpetsidaki ◽  
Vladimír Plicka ◽  
...  
Keyword(s):  
Numerical Models ◽  
Waveform Inversion ◽  
Strong Motion ◽  
Satellite System ◽  
Strike Slip ◽  
Plate Motion ◽  
Ocean Bottom ◽  
Ocean Bottom Seismometer ◽  
Motion Data ◽  
Global Navigation Satellite

Abstract With different styles of faulting, the eastern Ionian Sea is an ideal natural laboratory to investigate interactions between adjacent faults during strong earthquakes. The 2018 Mw 6.8 Zakynthos earthquake, well recorded by broadband and strong-motion networks, provides an opportunity to resolve such faulting complexity. Here, we focus on waveform inversion and backprojection of strong-motion data, partly checked by coseismic Global Navigation Satellite System data. We show that the region is under subhorizontal southwest–northeast compression, enabling mixed thrust faulting and strike-slip (SS) faulting. The 2018 mainshock consisted of two fault segments: a low-dip thrust, and a dominant, moderate-dip, right-lateral SS, both in the crust. Slip vectors, oriented to southwest, are consistent with plate motion. The sequence can be explained in terms of trench-orthogonal fractures in the subducting plate and reactivated faults in the upper plate. The 2018 event, and an Mw 6.6 event of 1997, occurred near three localized swarms of 2016 and 2017. Future numerical models of the slab deformation and ocean-bottom seismometer observations may illuminate possible relations among earthquakes, swarms, and fluid paths in the region.

Multi-parameter model building for the Qiuyue structure using four-component ocean-bottom seismometer data

Geophysics ◽  
2021 ◽  
pp. 1-52
Author(s):  
Yuzhu Liu ◽  
Xinquan Huang ◽  
Jizhong Yang ◽  
Xueyi Liu ◽  
Bin Li ◽  
...  
Keyword(s):  
Wave Velocity ◽  
Waveform Inversion ◽  
Velocity Model ◽  
P Wave ◽  
Velocity Analysis ◽  
Full Waveform Inversion ◽  
Ocean Bottom ◽  
Ocean Bottom Seismometer ◽  
Full Waveform ◽  
P Wave Velocity

Thin sand-mud-coal interbedded layers and multiples caused by shallow water pose great challenges to conventional 3D multi-channel seismic techniques used to detect the deeply buried reservoirs in the Qiuyue field. In 2017, a dense ocean-bottom seismometer (OBS) acquisition program acquired a four-component dataset in East China Sea. To delineate the deep reservoir structures in the Qiuyue field, we applied a full-waveform inversion (FWI) workflow to this dense four-component OBS dataset. After preprocessing, including receiver geometry correction, moveout correction, component rotation, and energy transformation from 3D to 2D, a preconditioned first-arrival traveltime tomography based on an improved scattering integral algorithm is applied to construct an initial P-wave velocity model. To eliminate the influence of the wavelet estimation process, a convolutional-wavefield-based objective function for the preprocessed hydrophone component is used during acoustic FWI. By inverting the waveforms associated with early arrivals, a relatively high-resolution underground P-wave velocity model is obtained, with updates at 2.0 km and 4.7 km depth. Initial S-wave velocity and density models are then constructed based on their prior relationships to the P-wave velocity, accompanied by a reciprocal source-independent elastic full-waveform inversion to refine both velocity models. Compared to a traditional workflow, guided by stacking velocity analysis or migration velocity analysis, and using only the pressure component or other single-component, the workflow presented in this study represents a good approach for inverting the four-component OBS dataset to characterize sub-seafloor velocity structures.

In-Situ Calibration of Differential Pressure Gauges on OBSIP Ocean Bottom Seismometers

2020 ◽  
Author(s):  
Gabi Laske ◽  
Adrian Doran
Keyword(s):  
Pressure Sensor ◽  
Broad Band ◽  
Pressure Gauge ◽  
Differential Pressure ◽  
P Wave ◽  
Earth Tides ◽  
Release Mechanism ◽  
Ocean Bottom ◽  
Ocean Bottom Seismometer

<p>A standard ocean bottom seismometer (OBS) package of the U.S. OBS Instrument Pool (OBSIP) carries a seismometer and a pressure sensor. For broadband applications, the seismometer typically is a wide-band or broad-band three-components seismometer, and the pressure sensor is a differential pressure gauge (DPG). The purpose of the pressure sensor is manifold and includes the capture of pressure signals not picked up by a ground motion sensor (e.g. the passage of tsunami), but also for purposes of correcting the seismograms for unwanted signals generated in the water column (e.g. p-wave reverberations).<br>Unfortunately, the instrument response of the widely used Cox-Webb DPG remains somewhat poorly known, and can vary by individual sensor, and even by deployment of the same sensor.</p><p>Efforts have been under way to construct and test DPG responses in the laboratory. But the sensitivity and long‐period response are difficult to calibrate as they  vary with temperature and pressure, and perhaps by hardware of the same mechanical specifications.  Here, we present a way to test the response for each individual sensor and deployment in situ in the ocean. This test requires a relatively minimal and inexpensive modification to the OBS instrument frame and a release mechanism that allows a drop of the DPG by 3 inches after the OBS package settled and the DPG equilibrated on the seafloor. The seismic signal generated by this drop is then analyzed in the laboratory upon retrieval of the data. </p><p>The results compare favorably with calibrations estimated independently through post‐deployment data analyses of other signals such as Earth tides and the signals from large teleseismic earthquakes. Our study demonstrates that observed response functions can deviate from the nominal response by a factor of two or greater with regards to both the sensitivity and the time constant. Given the fact that sensor calibrations of DPGs in the lab require very specific and stable environments and are time consuming, the use of in-situ DPG calibration frames pose a reliable and inexpensive alternative. </p>

Hybrid long-period volcanic events observed in off Nicobar region, the Andaman Sea from a passive OBS experiment

2020 ◽  
Author(s):  
Karanam Kattil Aswini ◽  
Pawan Dewangan ◽  
Kattoju Achuta Kamesh Raju ◽  
Yatheesh Vadakkeyakath ◽  
Pabitra Singha ◽  
...  
Keyword(s):  
High Frequency ◽  
Source Region ◽  
Strike Slip ◽  
Andaman Sea ◽  
Earthquake Swarms ◽  
Ocean Bottom ◽  
Upward Movement ◽  
Ocean Bottom Seismometer ◽  
Long Period ◽  
Back Arc

<p>The off Nicobar region in the Andaman Sea is witnessing frequent earthquake swarms after December 2004 Tsunamigenic earthquake in January 2005, March and October 2014, November 2015 and April 2019. In this study, we present the geophysical evidence of active volcanism in the Off Nicobar back-arc region on 21<sup>st</sup> and 22<sup>nd</sup> March 2014 based on a passive Ocean Bottom Seismometer (OBS) experiment. We detected a series of hybrid earthquake events characterized by the onset of high–frequency signal (1-10 Hz) which is followed by a long period waveform of up to 600s having a range of 0.1-1 Hz. The waveforms appear to be emergent and lack the onset of a distinct S-phase. We also observed a very high frequency (10-40 Hz) hydro-acoustic phase in the coda of long-period events.  These hybrid events are considered to be volcano-tectonic (VT) events that may trigger magmatic activities in the Off Nicobar region. We have identified and located 141 high-frequency events on 21<sup>st</sup> and 22<sup>nd</sup> March 2014 using hypocent v.3.2 program and they are distributed along NW-SE direction aligning with the submarine volcanoes defining the volcanic arc as observed in the high-resolution bathymetry data. The fault plane solution of the major high-frequency events suggests strike-slip faulting with the strike, dip and rake values of 334<sup>°</sup>, 89<sup>°</sup> and 171<sup>°</sup>, respectively along the direction of the prevalent sliver strike-slip faulting in the Andaman back-arc region. We propose that the upward movement of magma is a plausible mechanism which can explain the frequent occurrence of earthquake swarms in the off Nicobar region. The stress generated from magma movement may initially trigger shallow VT events such as faulting or dike intrusions and later generate long period ringing associated with the resonance of the magma chamber. The shallow nature of the events also generates a hydroacoustic wave which is detected in the OBS experiment as the source region is in the SOFAR channel.</p>

scholarly journals Crustal-scale depth imaging via joint full-waveform inversion of ocean-bottom seismometer data and pre-stack depth migration of multichannel seismic data: a case study from the eastern Nankai Trough

Solid Earth ◽  
2019 ◽  
Vol 10 (3) ◽  
pp. 765-784 ◽  
Author(s):  
Andrzej Górszczyk ◽  
Stéphane Operto ◽  
Laure Schenini ◽  
Yasuhiro Yamada
Keyword(s):  
Seismic Data ◽  
Nankai Trough ◽  
Waveform Inversion ◽  
Velocity Model ◽  
Full Waveform Inversion ◽  
Ocean Bottom ◽  
Ocean Bottom Seismometer ◽  
Depth Migration ◽  
Full Waveform ◽  
Air Gun

Abstract. Imaging via pre-stack depth migration (PSDM) of reflection towed-streamer multichannel seismic (MCS) data at the scale of the whole crust is inherently difficult. This is because the depth penetration of the seismic wavefield is controlled, firstly, by the acquisition design, such as streamer length and air-gun source configuration, and secondly by the complexity of the crustal structure. Indeed, the limited length of the streamer makes the estimation of velocities from deep targets challenging due to the velocity–depth ambiguity. This problem is even more pronounced when processing 2-D seismic data due to the lack of multi-azimuthal coverage. Therefore, in order to broaden our knowledge about the deep crust using seismic methods, we present the development of specific imaging workflows that integrate different seismic data. Here we propose the combination of velocity model building using (i) first-arrival tomography (FAT) and full-waveform inversion (FWI) of wide-angle, long-offset data collected by stationary ocean-bottom seismometers (OBSs) and (ii) PSDM of short-spread towed-streamer MCS data for reflectivity imaging, with the former velocity model as a background model. We present an application of such a workflow to seismic data collected by the Japan Agency for Marine-Earth Science and Technology (JAMSTEC) and the Institut Français de Recherche pour l'Exploitation de la Mer (IFREMER) in the eastern Nankai Trough (Tokai area) during the 2000–2001 Seize France Japan (SFJ) experiment. We show that the FWI model, although derived from OBS data, provides an acceptable background velocity field for the PSDM of the MCS data. From the initial PSDM, we refine the FWI background velocity model by minimizing the residual move-outs (RMOs) picked in the pre-stack-migrated volume through slope tomography (ST), from which we generate a better-focused migrated image. Such integration of different seismic datasets and leading-edge imaging techniques led to greatly improved imaging at different scales. That is, large to intermediate crustal units identified in the high-resolution FWI velocity model extensively complement the short-wavelength reflectivity inferred from the MCS data to better constrain the structural factors controlling the geodynamics of the Nankai Trough.

Soil Coupling Of a Strong Motion, Ocean Bottom Seismometer

1979 ◽  
Author(s):  
R.L. Steinmetz ◽  
P.L. Donoho ◽  
J.D. Murff ◽  
G.V. Latham
Keyword(s):  
Strong Motion ◽  
Ocean Bottom ◽  
Ocean Bottom Seismometer

scholarly journals Looking beyond kinematics: 3D thermo-mechanical modelling reveals the dynamics of transform margins

Solid Earth ◽  
2021 ◽  
Vol 12 (5) ◽  
pp. 1211-1232
Author(s):  
Anthony Jourdon ◽  
Charlie Kergaravat ◽  
Guillaume Duclaux ◽  
Caroline Huguen
Keyword(s):  
Boundary Conditions ◽  
Strain Localization ◽  
Numerical Models ◽  
Motion Vector ◽  
Shear Zones ◽  
Strike Slip ◽  
Plate Motion ◽  
Plate Boundaries ◽  
Mechanical Modelling ◽  
Extension Direction

Abstract. Transform margins represent ∼ 30 % of non-convergent margins worldwide. Their formation and evolution have traditionally been addressed through kinematic models that do not account for the mechanical behaviour of the lithosphere. In this study, we use high-resolution 3D numerical thermo-mechanical modelling to simulate and investigate the evolution of intra-continental strain localization under oblique extension. The obliquity is set through velocity boundary conditions that range from 15∘ (high obliquity) to 75∘ (low obliquity) every 15∘ for rheologies of strong and weak lower continental crust. Numerical models show that the formation of localized strike-slip shear zones leading to transform continental margins always follows a thinning phase during which the lithosphere is thermally and mechanically weakened. For low- (75∘) to intermediate-obliquity (45∘) cases, the strike-slip faults are not parallel to the extension direction but form an angle of 20∘ to 40∘ with the plate motion vector, while for higher obliquities (30∘ to 15∘) the strike-slip faults develop parallel to the extension direction. Numerical models also show that during the thinning of the lithosphere, the stress and strain re-orient while boundary conditions are kept constant. This evolution, due to the weakening of the lithosphere, leads to a strain localization process in three major phases: (1) initiation of strain in a rigid plate where structures are sub-perpendicular to the extension direction; (2) distributed deformation with local stress field variations and formation of transtensional and strike-slip structures; (3) formation of highly localized plate boundaries stopping the intra-continental deformation. Our results call for a thorough re-evaluation of the kinematic approach to studying transform margins.

Efficient and accurate modeling of ocean bottom seismometer data using reciprocity

Geophysics ◽  
2012 ◽  
Vol 77 (6) ◽  
pp. T211-T220
Author(s):  
Peyman Poor Moghaddam ◽  
Audun Libak ◽  
Henk Keers ◽  
Rolf Mjelde
Keyword(s):  
Order Approximation ◽  
Waveform Inversion ◽  
Synthetic Data ◽  
Staggered Grid ◽  
Elastic Model ◽  
Ocean Bottom ◽  
Ocean Bottom Seismometer ◽  
Full Waveform ◽  
Body Forces ◽  
Explosive Source

Seismic experiments in which the number of sources is considerably larger than the number of receivers occur regularly. An important example is the collection of crustal scale seismic data using ocean bottom seismometers and marine sources. We describe a method to accurately and efficiently compute synthetic seismograms for such experiments by using finite differences and reciprocity. We show numerically how to decompose an explosive source into its equivalent body force components using the staggered-grid finite-difference technique with a fourth-order approximation for the spatial derivative and a second-order approximation for the temporal derivative. This decomposition results in a source configuration where the equivalent body forces are defined in 12 points around the point where the ex-plosive source is applied. We then use the derived equivalent body forces for the explosive source and seismic reciprocity theorems to convert the common receiver gather to a common shot gather. The method is tested on a structurally complex elastic model of the crust and the results show that it is accurate within floating point precision. The synthetic data are compared to data from a real ocean bottom seismometer experiment conducted across a continent-ocean transition zone. A good fit in terms of traveltime is observed for many of the prominent seismic phases. The amplitude fit of these arrivals is not always as good as the traveltime fit. This indicates that full-waveform modeling of such data can provide useful information about the subsurface that cannot be obtained from traveltime modeling. If enough data are available, the modeling method can be used in full-waveform inversion.

The Alaska Amphibious Community Seismic Experiment

2020 ◽  
Vol 91 (6) ◽  
pp. 3054-3063 ◽  
Author(s):  
Grace Barcheck ◽  
Geoffrey A. Abers ◽  
Aubreya N. Adams ◽  
Anne Bécel ◽  
John Collins ◽  
...  
Keyword(s):  
Strong Motion ◽  
Seismic Survey ◽  
Absolute Pressure ◽  
Ocean Bottom ◽  
Ocean Bottom Seismometer ◽  
Alaska Peninsula ◽  
Outer Rise ◽  
Kodiak Island ◽  
Active Source ◽  
Seismic Experiment

Abstract The Alaska Amphibious Community Seismic Experiment (AACSE) is a shoreline-crossing passive- and active-source seismic experiment that took place from May 2018 through August 2019 along an ∼700  km long section of the Aleutian subduction zone spanning Kodiak Island and the Alaska Peninsula. The experiment featured 105 broadband seismometers; 30 were deployed onshore, and 75 were deployed offshore in Ocean Bottom Seismometer (OBS) packages. Additional strong-motion instruments were also deployed at six onshore seismic sites. Offshore OBS stretched from the outer rise across the trench to the shelf. OBSs in shallow water (<262  m depth) were deployed with a trawl-resistant shield, and deeper OBSs were unshielded. Additionally, a number of OBS-mounted strong-motion instruments, differential and absolute pressure gauges, hydrophones, and temperature and salinity sensors were deployed. OBSs were deployed on two cruises of the R/V Sikuliaq in May and July 2018 and retrieved on two cruises aboard the R/V Sikuliaq and R/V Langseth in August–September 2019. A complementary 398-instrument nodal seismometer array was deployed on Kodiak Island for four weeks in May–June 2019, and an active-source seismic survey on the R/V Langseth was arranged in June 2019 to shoot into the AACSE broadband network and the nodes. Additional underway data from cruises include seafloor bathymetry and sub-bottom profiles, with extra data collected near the rupture zone of the 2018 Mw 7.9 offshore-Kodiak earthquake. The AACSE network was deployed simultaneously with the EarthScope Transportable Array (TA) in Alaska, effectively densifying and extending the TA offshore in the region of the Alaska Peninsula. AACSE is a community experiment, and all data were made available publicly as soon as feasible in appropriate repositories.

scholarly journals Seafloor sediment thickness beneath the VoiLA broad-band ocean-bottom seismometer deployment in the Lesser Antilles from P-to-S delay times

2020 ◽  
Vol 223 (3) ◽  
pp. 1758-1768
Author(s):  
Ben Chichester ◽  
Catherine Rychert ◽  
Nicholas Harmon ◽  
Robert Allen ◽  
Jenny Collier ◽  
...  
Keyword(s):  
Velocity Structure ◽  
Confidence Region ◽  
Broad Band ◽  
Lesser Antilles ◽  
Earth Model ◽  
Ocean Bottom ◽  
Ocean Bottom Seismometer ◽  
Sediment Thickness ◽  
Delay Times ◽  
Crystalline Crust

SUMMARY Broad-band ocean-bottom seismometer (OBS) deployments present an opportunity to investigate the seafloor sediment thickness, which is important for constraining sediment deposition, and is also useful for subsequent seismological analyses. The Volatile Recycling in the Lesser Antilles (VoiLA) project deployed 34 OBSs over the island arc, fore- and backarc of the Lesser Antilles subduction zone for 15 months from 2016 to 2017. Using the amplitudes and delay times of P-to-S (Ps) scattered waves from the conversion of teleseismic earthquake Pwaves at the crust–sediment boundary and pre-existing relationships developed for Cascadia, we estimate sediment thickness beneath each OBS. The delay times of the Ps phases vary from 0.20 ± 0.06 to 3.55 ± 0.70 s, generally increasing from north to south. Using a single-sediment and single-crystalline crust earth model in each case, we satisfactorily model the observations of eight OBSs. At these stations we find sediment thicknesses range from 0.43 ± 0.45 to 5.49 ± 3.23 km. To match the observations of nine other OBSs, layered sediment and variable thickness crust is required in the earth model to account for wave interference effects on the observed arrivals. We perform an inversion with a two-layer sediment and a single-layer crystalline crust in these locations finding overall sediment thicknesses of 1.75 km (confidence region: 1.45–2.02 km) to 7.93 km (confidence region: 6.32–11.05 km), generally thinner than the initial estimates based on the pre-existing relationships. We find agreement between our modelled velocity structure and the velocity structure determined from the VoiLA active-source seismic refraction experiment at the three common locations. Using the Ps values and estimates from the VoiLA refraction experiment, we provide an adjusted relationship between delay time and sediment equations for the Lesser Antilles. Our new relationship is ${{H}} = {{1.42}}{{\rm d}}{{{t}}^{ {1.44}}}$ , where H is sediment thickness in kilometres and dt is mean observed Ps delay time in seconds, which may be of use in other subduction zone settings with thick seafloor sediments.

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